---
layout: post
title:  "gem5-22.0 Released!"
author: Jason Lowe-Power
date:   2022-06-18
categories: project
---

First of all, thank you to all of the contributors who made this another great gem5 release!
We'll be talking about this release and many other cool things that have happened in the gem5 community over the past few years at the [gem5 workshop with ISCA 2022](https://www.gem5.org/events/isca-2022#the-4th-gem5-users-workshop).
You can find a livestream and recording [on our youtube channel](https://www.youtube.com/channel/UCCpCGEj_835WYmbB0g96lZw).

gem5 version 22.0 has been slightly delayed, but we a have a very strong release!
This release has 660 changes from 48 unique contributors.
While there are not too many big ticket features, the community has done a lot to improve the stablity and add bugfixes to gem5 over this release.
That said, we have a few cool new features like full system GPU support, a huge number of Arm improvements, and an improved HBM model.

See below for more details!

## New features

- [Arm now models DVM messages for TLBIs and DSBs accurately](https://gem5.atlassian.net/browse/GEM5-1097). This is implemented in the CHI protocol.
- EL2/EL3 support on by default in ArmSystem
- HBM controller which supports pseudo channels
- [Improved Ruby's SimpleNetwork routing](https://gem5.atlassian.net/browse/GEM5-920)
- Added x86 bare metal workload and better real mode support
- [Added round-robin arbitration when using multiple prefetchers](https://gem5.atlassian.net/browse/GEM5-1169)
- [KVM Emulation added for ARM GIGv3](https://gem5.atlassian.net/browse/GEM5-1138)
- Many improvements to the CHI protocol

## Many RISC-V instructions added

The following RISCV instructions have been added to gem5's RISC-V ISA:

* Zba instructions: add.uw, sh1add, sh1add.uw, sh2add, sh2add.uw, sh3add, sh3add.uw, slli.uw
* Zbb instructions: andn, orn, xnor, clz, clzw, ctz, ctzw, cpop, cpopw, max, maxu, min, minu, sext.b, sext.h, zext.h, rol, rolw, ror, rori, roriw, rorw, orc.b, rev8
* Zbc instructions: clmul, clmulh, clmulr
* Zbs instructions: bclr, bclri, bext, bexti, binv, binvi, bset, bseti
* Zfh instructions: flh, fsh, fmadd.h, fmsub.h, fnmsub.h, fnmadd.h, fadd.h, fsub.h, fmul.h, fdiv.h, fsqrt.h, fsgnj.h, fsgnjn.h, fsgnjx.h, fmin.h, fmax.h, fcvt.s.h, fcvt.h.s, fcvt.d.h, fcvt.h.d, fcvt.w.h, fcvt.h.w, fcvt.wu.h, fcvt.h.wu

### Improvements to the stdlib automatic resource downloader

The gem5 standard library's downloader has been re-engineered to more efficiently obtain the `resources.json` file.
It is now cached instead of retrieved on each resource retrieval.

The `resources.json` directory has been moved to a more permament URL at <http://resources.gem5.org/resources.json>.

Tests have also been added to ensure the resources module continues to function correctly.

### gem5 in SystemC support revamped

The gem5 in SystemC has been revamped to accomodate new research needs.
These changes include stability improvements and bugs fixes.
The gem5 testing suite has also been expanded to include gem5 in SystemC tests.

### Improved GPU support.

Users may now simulate an AMD GPU device in full system mode using the ROCm 4.2 compute stack.
Until v21.2, gem5 only supported GPU simulation in Syscall-Emulation mode with ROCm 4.0.
See [`src/gpu-fs/README.md`](https://gem5.googlesource.com/public/gem5-resources/+/refs/heads/stable/src/gpu-fs/) in gem5-resources and example scripts in [`configs/example/gpufs/`](https://gem5.googlesource.com/public/gem5/+/refs/tags/v22.0.0.0/configs/example/gpufs/) for example scripts which run GPU full system simulations.

A [GPU Ruby random tester has been added](https://gem5-review.googlesource.com/c/public/gem5/+/59272) to help validate the correctness of the CPU and GPU Ruby coherence protocols as part of every kokoro check-in.
This helps validate the correctness of the protocols before new changes are checked in.
Currently the tester focuses on the protocols used with the GPU, but the ideas are extensible to other protocols.
The work is based on "Autonomous Data-Race-Free GPU Testing", IISWC 2019, Tuan Ta, Xianwei Zhang, Anthony Gutierrez, and Bradford M. Beckmann.

### An Arm board has been added to the gem5 Standard Library

Via [this change](https://gem5-review.googlesource.com/c/public/gem5/+/58910), an ARM Board, `ArmBoard`, has been added to the gem5 standard library.
This allows for an ARM system to be run using the gem5 stdlib components.

An example gem5 configuration script using this board can be found in `configs/example/gem5_library/arm-ubuntu-boot-exit.py`.

### `createAddrRanges` now supports NUMA configurations

When the system is configured for NUMA, it has multiple memory ranges, and each memory range is mapped to a corresponding NUMA node. For this, the change enables `createAddrRanges` to map address ranges to only a given HNFs.

Jira ticker here: https://gem5.atlassian.net/browse/GEM5-1187.

## API (user-facing) changes

### CPU model types are no longer simply the model name, but they are specialized for each ISA

For instance, the `O3CPU` is now the `X86O3CPU` and `ArmO3CPU`, etc.
This requires a number of changes if you have your own CPU models.
See https://gem5-review.googlesource.com/c/public/gem5/+/52490 for details.

Additionally, this requires changes in any configuration script which inherits from the old CPU types.

In many cases, if there is only a single ISA compiled the old name will still work.
However, this is not 100% true.

Finally, `CPU_MODELS` is no longer a parameter in `build_opts/`.
Now, if you want to compile a CPU model for a particular ISA you will have to add a new file for the CPU model in the `arch/` directory.

### Many changes in the CPU and ISA APIs

If you have any specialized CPU models or any ISAs which are not in the mainline, expect many changes when rebasing on this release.

- No longer use read/setIntReg (e.g., see https://gem5-review.googlesource.com/c/public/gem5/+/49766)
- InvalidRegClass has changed (e.g., see https://gem5-review.googlesource.com/c/public/gem5/+/49745)
- All of the register classes have changed (e.g., see https://gem5-review.googlesource.com/c/public/gem5/+/49764/)
- `initiateSpecialMemCmd` renamed to `initiateMemMgmtCmd` to generalize to other command beyond HTM (e.g., DVM/TLBI)
- `OperandDesc` class added (e.g., see https://gem5-review.googlesource.com/c/public/gem5/+/49731)
- Many cases of `TheISA` have been removed

## Bug Fixes

- [Fixed RISC-V call/ret instruction decoding](https://gem5-review.googlesource.com/c/public/gem5/+/58209). The fix adds IsReturn` and `IsCall` flags for RISC-V jump instructions by defining a new `JumpConstructor` in "standard.isa". Jira Ticket here: https://gem5.atlassian.net/browse/GEM5-1139.
- [Fixed x86 Read-Modify-Write behavior in multiple timing cores with classic caches](https://gem5-review.googlesource.com/c/public/gem5/+/55744). Jira Ticket here: https://gem5.atlassian.net/browse/GEM5-1105.
- [The circular buffer for the O3 LSQ has been fixed](https://gem5-review.googlesource.com/c/public/gem5/+/58649). This issue affected running the O3 CPU with large workloaders. Jira Ticket here: https://gem5.atlassian.net/browse/GEM5-1203.
- [Removed "memory-leak"-like error in RISC-V lr/sc implementation](https://gem5-review.googlesource.com/c/public/gem5/+/55663). Jira issue here: https://gem5.atlassian.net/browse/GEM5-1170.
- [Resolved issues with Ruby's memtest](https://gem5-review.googlesource.com/c/public/gem5/+/56811). In gem5 v21.2, If the size of the address range was smaller than the maximum number of outstandnig requests allowed downstream, the tester would get stuck trying to find a unique address. This has been resolved.

## Build-related changes

- Variable in `env` in the SConscript files now requires you to use `env['CONF']` to access them. Anywhere that `env['<VARIABLE>']` appeared should noe be `env['CONF']['<VARIABLE>']`
- Internal build files are now in a per-target `gem5.build` directory
- All build variable are per-target and there are no longer any shared variables.

## Other changes

- New bootloader is required for Arm VExpress_GEM5_Foundation platform. See https://gem5.atlassian.net/browse/GEM5-1222 for details.
- The MemCtrl interface has been updated to use more inheritance to make extending it to other memory types (e.g., HBM pseudo channels) easier.